Which of these numbers is prime? ${7,\ 25,\ 60,\ 75,\ 87}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 7 are 1 and 7. The factors of 25 are 1, 5, and 25. The factors of 60 are 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 87 are 1, 3, 29, and 87. Thus, 7 is a prime number.